uapi headers: simplify the <linux/compiler.h> stuff.

We don't really need <linux/compiler.h> and <linux/compiler_types.h>. We
already have a mechanism to remove unused macros, so let's do that. We
don't currently have a way to remove unused #includes, so we still need
<linux/compiler.h> and <linux/compiler_types.h> files (but I've clarified
the comments in them).

I've kept the empty definitions of `__user` and `__force` for source
compatibility. (We had one security test at least that was assuming
a kernel struct definition will "just work".)

Bug: http://b/262917450
Test: treehugger
Change-Id: Iacbbbc1aeef9a4fac52dabd7811ab875cc267d4f
diff --git a/libc/kernel/uapi/linux/videodev2.h b/libc/kernel/uapi/linux/videodev2.h
index d418fd2..4fb0a25 100644
--- a/libc/kernel/uapi/linux/videodev2.h
+++ b/libc/kernel/uapi/linux/videodev2.h
@@ -650,7 +650,7 @@
 #define V4L2_FBUF_FLAG_SRC_CHROMAKEY 0x0040
 struct v4l2_clip {
   struct v4l2_rect c;
-  struct v4l2_clip __user * next;
+  struct v4l2_clip  * next;
 };
 struct v4l2_window {
   struct v4l2_rect w;
@@ -658,7 +658,7 @@
   __u32 chromakey;
   struct v4l2_clip * clips;
   __u32 clipcount;
-  void __user * bitmap;
+  void  * bitmap;
   __u8 global_alpha;
 };
 struct v4l2_captureparm {
@@ -895,32 +895,32 @@
   union {
     __s32 value;
     __s64 value64;
-    char __user * string;
-    __u8 __user * p_u8;
-    __u16 __user * p_u16;
-    __u32 __user * p_u32;
-    __u32 __user * p_s32;
-    __u32 __user * p_s64;
-    struct v4l2_area __user * p_area;
-    struct v4l2_ctrl_h264_sps __user * p_h264_sps;
+    char  * string;
+    __u8  * p_u8;
+    __u16  * p_u16;
+    __u32  * p_u32;
+    __u32  * p_s32;
+    __u32  * p_s64;
+    struct v4l2_area  * p_area;
+    struct v4l2_ctrl_h264_sps  * p_h264_sps;
     struct v4l2_ctrl_h264_pps * p_h264_pps;
-    struct v4l2_ctrl_h264_scaling_matrix __user * p_h264_scaling_matrix;
-    struct v4l2_ctrl_h264_pred_weights __user * p_h264_pred_weights;
-    struct v4l2_ctrl_h264_slice_params __user * p_h264_slice_params;
-    struct v4l2_ctrl_h264_decode_params __user * p_h264_decode_params;
-    struct v4l2_ctrl_fwht_params __user * p_fwht_params;
-    struct v4l2_ctrl_vp8_frame __user * p_vp8_frame;
-    struct v4l2_ctrl_mpeg2_sequence __user * p_mpeg2_sequence;
-    struct v4l2_ctrl_mpeg2_picture __user * p_mpeg2_picture;
-    struct v4l2_ctrl_mpeg2_quantisation __user * p_mpeg2_quantisation;
-    struct v4l2_ctrl_vp9_compressed_hdr __user * p_vp9_compressed_hdr_probs;
-    struct v4l2_ctrl_vp9_frame __user * p_vp9_frame;
-    struct v4l2_ctrl_hevc_sps __user * p_hevc_sps;
-    struct v4l2_ctrl_hevc_pps __user * p_hevc_pps;
-    struct v4l2_ctrl_hevc_slice_params __user * p_hevc_slice_params;
-    struct v4l2_ctrl_hevc_scaling_matrix __user * p_hevc_scaling_matrix;
-    struct v4l2_ctrl_hevc_decode_params __user * p_hevc_decode_params;
-    void __user * ptr;
+    struct v4l2_ctrl_h264_scaling_matrix  * p_h264_scaling_matrix;
+    struct v4l2_ctrl_h264_pred_weights  * p_h264_pred_weights;
+    struct v4l2_ctrl_h264_slice_params  * p_h264_slice_params;
+    struct v4l2_ctrl_h264_decode_params  * p_h264_decode_params;
+    struct v4l2_ctrl_fwht_params  * p_fwht_params;
+    struct v4l2_ctrl_vp8_frame  * p_vp8_frame;
+    struct v4l2_ctrl_mpeg2_sequence  * p_mpeg2_sequence;
+    struct v4l2_ctrl_mpeg2_picture  * p_mpeg2_picture;
+    struct v4l2_ctrl_mpeg2_quantisation  * p_mpeg2_quantisation;
+    struct v4l2_ctrl_vp9_compressed_hdr  * p_vp9_compressed_hdr_probs;
+    struct v4l2_ctrl_vp9_frame  * p_vp9_frame;
+    struct v4l2_ctrl_hevc_sps  * p_hevc_sps;
+    struct v4l2_ctrl_hevc_pps  * p_hevc_pps;
+    struct v4l2_ctrl_hevc_slice_params  * p_hevc_slice_params;
+    struct v4l2_ctrl_hevc_scaling_matrix  * p_hevc_scaling_matrix;
+    struct v4l2_ctrl_hevc_decode_params  * p_hevc_decode_params;
+    void  * ptr;
   };
 } __attribute__((packed));
 struct v4l2_ext_controls {